(1) Field of the Invention
This invention relates to a method and an assembly for practicing the method of molding hollow thin-walled articles from a heat-fusing material such as vinyl plastisol by slush molding.
Various vinyl plastisol compositions are known and used in various slush molding methods in various assemblies for performing those methods. Typically, an open hollow mold is filled with liquid plastisol and heat is applied to the mold surface and transferred to the liquid to gel a layer of plastisol adjacent the mold surface. The thickness of the gelled layer depends upon the temperature to which the plastisol layer is subjected and the time the plastisol is maintained at that temperature. After the layer adjacent the mold surface is gelled, the ungelled plastisol is dumped or poured out of the mold.
(2) Description of the Prior Art
In one well-known method, the mold temperature is maintained relatively low to prevent gelling as a very thin initial layer or coating of plastisol is applied to the mold to prevent bare spots and to prevent the entrapment of air bubbles, thereby providing a thin coating which strictly conforms to the mold configuration. Such a coating is applied by completely filling and completely emptying the mold except for the coated layer of material. For example, automotive dashboard or instrument panel covers are made in this fashion and the molds frequently include details to define imitation stitching in the finished product as well as undercuts and other intricate detail. A very thin coating is first applied to conform to the mold and to prevent irregularities in the outer surface of the finished product. After this very thin coating is applied, the mold is dumped and is again filled with additional plastisol and the entire mold is heated to gel the plastisol and increase the thickness of the finished product. After the desired thickness is attained, the mold is again dumped or emptied and thereafter the mold is subjected to additional heat for curing the product.
An example of such a method and an assembly for practicing same is shown in the U.S. Pat. No. 3,728,429 granted to Daniel E. Colby, Philip E. Rogers and Frederick J. Sliwinski on Apr. 17, 1973 and assigned to the assignee of the subject invention. That patent discloses such a method for slush molding articles wherein an endless conveyor moves a plurality of molds through various stations in the performance of the method. Each mold is separately filled with liquid plastisol to cover the mold surfaces and the fill is later dumped. U.S. Pat. No. 4,217,325 issued Aug. 12, 1980 discloses a single station apparatus more suited to short runs of parts and to quick and efficient changeovers from one plastisol to another. Although the methods and assemblies shown in these patents have proved to be very satisfactory, one disadvantage is that each of the molds is completely filled with liquid and the fill cycle time is established by the mold volume rather than the quantity of material deposited on the mold surfaces.
In the slush process currently employed for large parts, viz "modular - slush", a significant portion of the cycle time is taken up by filling the empty mold in some cases with as much as 300 pounds of plastisol and dumping more than 95% of this material back out. Only a small portion of the vinyl plastisol adduct is effectively used; the rest is process recycle. In a sense, the process can be visualized as an approximate 5% conversion process.
The fill time cycle is in part exacerbated because plastisol is a highly viscous material and cannot flow or be pumped at high speed. Further, the total wall area of the tool must be covered in order to make a skin with total integrity. Finally, the mold tools are often deep, resembling in some cases a contoured bathtub.
An object of the subject invention is to provide an improved method and assembly for reducing the fill cycle time in a process for producing a thickness of an article in a slush molding process in a mold of the open-ended type.
Another object is to provide an open-ended liquid plastisol mold associated with an insert disposed adjacent the mold surface where the material is deposited to form the finished article. A flow space between the mold surface and the insert is supplied with liquid to cover the mold surface with liquid plastisol to define a coating or layer. The volume of the flow space is substantially less than the total volume of the mold cavity which defines the mold surface to be coated. Consequently, the fill cycle time i considerably reduced. The flow space, however, enables a very thin coating of plastisol to be free of surface blemishes.
While the method and apparatus of U.S. Pat. No. 4,217,325 covers mold surfaces with a coating of plastisol, it requires that the mold be completely filled with liquid. The fill cycle time is thus dependent on the open mold volume required to define the surface of the part to be molded.